Integrated circuits (ICs) generate heat. To prevent excessive temperatures, this heat must be dissipated through the IC package that houses the IC. This heat must also be dissipated through supporting hardware. Packages used in avionics or space applications are typically cooled by conduction from the bottom of a package into a printed wiring board (PWB).
FIG. 1 shows a package 10, comprising lid 12 and a housing body 14. Seated within the housing body 14 is an IC 16. The housing body is mounted to a PWB 18, via pins, solder balls, or solder columns 20. Packages in avionics or space applications are typically cooled by conduction into the PWB as opposed to convection cooling. Many high pincount packages use pins or balls on the bottom of a package for electrical connection to a PWB, which increases the thermal resistance between the package and PWB compared to perimeter leaded packages with full bottom side contact to the PWB. Furthermore, as PWB complexity increases, the number of layers in the PWB increases, and the heat from the package must be conducted through multiple layers (often through vias) before being conducted to the edge of the board, which further increases the thermal resistance of this conduction path. To alleviate these problems, emerging hardware designs attempt to remove heat from the top of the package through heat sinks, heat pipes, cooling plates, or other thermally conducting structures.
Unfortunately, standard hermetically-sealed ceramic packages used in avionics or space applications, have a high thermal resistance or a poor thermal conduction path from the IC to the top side of the package. There is also a large air gap that exists between the IC and the lid, further impending heat conduction. Generally speaking, the only conduction path to the top of the package is laterally through the package to the lid, and laterally across the lid.
FIGS. 2A-C respectively show prior art IC packages 22a-c. Each of the packages comprises a housing body 24 and a lid 26. Such housing bodies are generally made of alumina ceramic, which is a low thermal conductivity material (approx. 14 W/mK), and typical lids and seal rings are made of iron-nickel-cobalt alloy (ASTM F-15), which is a low thermal conductivity metal (approximately 17 W/mK). In addition, there is a small area of contact 27 between the package and lid (usually a solder seal or weld), and lids are thin, so the cross-sections for thermal conduction are small. Thus, these typical features result in high thermal resistance paths 28 and 30, between an IC and the top side or lid of a package.